Fix variable name typo reported by Alan Wilter by email

http://lists.open-bio.org/pipermail/biopython/2014-January/008918.html

Bio/SeqIO/SeqXmlIO.py

@@ -246,7 +246,7 @@ def write_header(self):
         if self.source is not None:
             attrs["source"] = self.source
         if self.source_version is not None:
-            attrs["sourceVersion"] = self.source_ersion
+            attrs["sourceVersion"] = self.source_version
         if self.species is not None:
             if not isinstance(species, basestring):
                 raise TypeError("species should be of type string")